In many applications of fluid flow control it is desirable that a relatively large rate of flow of fluid to the load be provided. The flow normally available from a flow control valve utilizing a spool valve as the second stage is limited by the perimeter of the spool, that is, the slot width through which the fluid must flow must be equal to or less than Pi times the spool diameter. Therefore, in the prior art, if such high flow rates are required, the solution has been to increase the diameter of the spool to thereby increase the slot size and thus the fluid flow rate.
Clearly, if such an approach is taken, the size of the control valve is substantially increased and under many applications such cannot be tolerated because of the limitations regarding space and weight.
The best prior art known to applicant are the following United States Letters Patents:
U.S. Pat. No. 2,816,420 PA1 U.S. Pat. No. 2,994,345 PA1 U.S. Pat. No. 3,070,124 PA1 U.S. Pat. No. 3,472,281 PA1 U.S. Pat. No. 3,477,472 PA1 U.S. Pat. No. 3,556,155 PA1 U.S. Pat. No. 3,561,488 PA1 U.S. Pat. No. 3,563,272 PA1 U.S. Pat. No. 3,580,281 PA1 U.S. Pat. No. 4,319,609